Ferritic stainless steel is being used in broad fields such as kitchen equipment, home electrical appliances, electronic devices, etc. However, compared with austenitic stainless steel, it is inferior in workability, so it is limited to these applications in some cases. In recent years, improvements in refining techniques in the production of ferrous metals have enabled reduction of carbon and nitrogen to extremely low levels and reduction of Si plus reduction of P, S, and other impurity elements. Ferritic stainless steel improved in workability by addition of Ti and other stabilizing elements (below, “high purity ferritic stainless steel”) is consequently being used for broader applications. This is because ferritic stainless steel is more economical compared with austenitic stainless steel—which contains a large amount of Ni—an element whose price has skyrocketed in recent years.
High purity ferritic stainless steel, as will be understood from SUS430LX standardized by the JIS, is often lower in amount of Cr and has issues in corrosion resistance compared with the typical austenitic stainless steel SUS304 (18Cr-8Ni). Further, for stainless steel sinks and other kitchen equipment and home electrical appliances where a good appearance is sought, the pitting, rusting, and other deterioration in surface properties due to corrosion are often problems.
To solve the above problem of corrosion resistance, there are the method of alloying Cr, Mo, etc. and the method of reforming a coating film formed on the steel surface by bright annealing. The former invites a rise in cost due to the alloying and becomes a factor inhibiting workability, so is not preferred. The latter is an effective method from the perspectives of suppressing a rise in material costs and a drop in workability. Various inventions have been proposed relating to the improvement of the coating film using bright annealing.
From the latter viewpoint, the inventors, in Japanese Patent Application No. 2006-172489, proposed a bright annealed ferritic stainless steel sheet with excellent rustproofness and workability having a ratio of Cr/Fe concentration in the coating film of over 0.5 and containing TiO2 in the coating film and a method of production of the same, but steel with a coating film improved using bright annealing has a problem in terms of securing corrosion resistance at a new surface exposed by working or later polishing or grinding. Measures against this problem were not described in the above patent application.
Further, as a method for solving this problem, it may be considered to utilize trace elements to improve the corrosion resistance. Japanese Patent Publication (A) No. 6-172935 and Japanese Patent Publication (A) No. 7-34205 disclose ferritic stainless steels intentionally adding P to improve the weatherability, rustproofness, and crevice corrosion resistance. Japanese Patent Publication (A) No. 6-172935 is high Cr and P ferritic stainless steel containing Cr over 20% to 40% and P over 0.06% to 0.2%. Japanese Patent Publication (A) No. 7-34205 is P ferritic stainless steel containing Cr 11% to less than 20% and P over 0.04% to 0.2%. However, P becomes a factor inhibiting manufacturability, workability, and weldability, so is not suited for applications where workability is demanded.
Furthermore, Japanese Patent Publication (A) No. 2000-169943 discloses ferritic stainless steel superior in high temperature strength containing trace amounts of Sn and Sb and a method of production of the same. The majority of the steels shown in the examples of Japanese Patent Publication (A) No. 2000-169943 are Cr 10 to 12% low Cr steels. In high Cr steels with Cr over 12%, V, Mo, etc. are added together to secure high temperature strength. As the effects of Sn and Sb, improvement of the high temperature strength may be mentioned. Whether sufficient corrosion resistance can be secured is not disclosed, so remains a question.
Japanese Patent Publication (A) No. 2001-288543 and Japanese Patent Publication (A) No. 2001-288544 disclose ferritic stainless steel with excellent surface characteristics and corrosion resistance using Mg and Ca as trace elements and a method of production of the same. Sn is a selectively added element and is described as an element preferable for the corrosion resistance. The steels shown in the examples of these Japanese Patent Publication (A) No. 2001-288543 and Japanese Patent Publication (A) No. 2001-288544 have Sn and expensive Co added to them together. These steels are 11.6% Cr steels or 16% Cr steels containing large amounts of C and other impurity elements. The pitting potentials are described as being respectively 0.086V and 0.12V. The pitting potentials are lower compared with the pitting potential (over 0.2V) equivalent to SUS304 targeted by the present invention.
Furthermore, WO2007/129703 has as its object the improvement of the pitting lifetime of auto parts etc. and discloses ferritic stainless steel with excellent crevice corrosion resistance using Sn and Sb as trace elements. The steels shown in the examples of this WO2007/129703 improve the pitting resistance at crevice parts by composite addition of Sn and Ni in almost all cases. 16% Cr steel in which Sn is added alone had a high amount of Si and did not correspond to high purity ferritic stainless steel covered by the present invention.
As explained above, the conventional art for improvement of the corrosion resistance utilizing trace elements did not cover addition of P alone or composite addition of Sn or Sb with the expensive rare element Co or Ni or cover the high purity ferritic stainless steel described in lines 13-29 on page 2 and had issues from the viewpoint of manufacturability, workability, and material costs.